In many cases, you can choose your topic based on the area of expertise you have. If you're a known (or unknown?) expert on RAID, you can give a presentation on RAID. But what do you talk about on RAID? Do you talk about the design of the controller drivers under Linux 2.4? Do you talk about installing Linux on a RAID device? Case study?

Unfortunately, these are things you'll have to think about and decide on your own. Your best bet is to think of a generic enough subject, and change it based on your audience (we'll get into this later on). If you're talking to a more advanced LUG/USENIX crowd, you can give a more advanced presentation, whereas newbie groups can get a presentation of what RAID is and how it works. The basic idea is to keep the intended audience in mind at all times.